This invention relates to a phased array antenna and, more particularly, to a phased array antenna composed of radiators having plural ports for introduction of orthogonally polarized radiation to individual ones of the radiators.
Phased array antennas are widely used for directing one or more beams of radiation in desired directions for transmission of radiant energy and for reception of radiant energy. Such antennas are used, by way of example, in satellite communication systems and in aircraft guidance systems. The antennas are useful because beam steering and beam pattern reconfiguration can be performed electronically, and without moving parts. In a typical phased array antenna, there are a plurality of radiators, each of which serves as an element of the antenna. It has been the practice to construct each radiator with a single port coupled electromagnetically to a signal means, wherein the signal means is a transmitting amplifier in the case of an antenna which transmits a beam of radiation, the signal means being a receiving amplifier in the case of an antenna which receives an incoming electromagnetic signal. The operation of a phased array antenna in the transmission mode is essentially the same as the operation in a receiving mode except that the direction of signal flow is reversed between the two modes.
By way of example, in the case of the receiving mode, a plurality of the radiators receives radiated signals with a specified polarization from a wide range of far field angles. The signal received at the individual radiators are then amplified, phase shifted, attenuated, and summed to produce a final antenna output. The phased array antenna can produce a narrow beam by virtue of the fact that only signals in a desired far field direction will add up in phase to produce a large output signal. A pointing of the beam is accomplished by adjustment of the phase shifters to cancel increments of phase shift experienced by successive ones of the radiators of the array from an incoming signal wavefront angled relative to the array of radiators. The attenuators are utilized to shape the beam pattern, as well as for calibration purposes. Multiple beams can be generated from the same radiating aperture of the antenna by adding more phase shifters and attenuators for each antenna element, or radiator, to produce several summed outputs.
A problem arises with presently available phased array antennas in that there is only one output port, or input port, provided for each antenna element. Therefore, the polarization properties of the phased array antenna are determined by the polarization properties of the individual antenna elements. This produces a disadvantage in that the polarization properties of the antenna cannot be programmed spatially otherwise. A further disadvantage is that the polarization orthogonality properties are determined by imperfections which may be present in the individual radiators, a disadvantage which is particularly significant for a wide field of view. Due to the fact that the polarization property of the antenna depends on the design of the individual radiators, such antennas have suffered from the limitation that only one polarization can be obtained over a complete field of view for each beam, and a further limitation that it is difficult to maintain good polarization orthogonality properties over a large field of view.